A gas turbine combustor includes: a head end comprising a primary fuel nozzle; a liner defining a primary combustion zone proximate the head end and a downstream secondary combustion zone; a forward casing radially outward of and surrounding at least a portion of the liner; and an axial fuel staging system. The axial fuel staging system includes a plurality of fuel injection assemblies. Each fuel injection assembly includes a thimble assembly mounted to the liner, and an injector unit attached to the forward casing. A thimble of the thimble assembly extends through a thimble aperture in the liner. The injector unit extends through the forward casing, such that a portion of the injector unit is disposed within the thimble, and a fuel line fitting is disposed outward of the forward casing. The injector unit introduces fuel into air flowing through the thimble for injection into the secondary combustion zone.